An optical fiber ribbon is a well-known structure that includes a plurality of individual optical fibers held in spaced positions adjacent to one another, typically in a flat or planar configuration. Ribbons are generally constructed by aligning two or more optical fibers side-by-side and extruding a radiation-curable matrix material over them. The matrix material is a pliable and protective material, such as a polyurethane acrylate resin, which encases the fibers and holds them in their ribbon-like arrangement.
Unlike discrete optical fibers, which may be difficult to handle or splice, the optical fiber ribbon provides a modular design that simplifies installation and maintenance. For example, rather than having to splice hundreds of individual thread-like fibers in the field, an installer can use a fusion splicer to make connections at once involving all or subgroups of optical fibers from the ribbon. The simplicity from ribbons is magnified when they are collected in large quantities in an optical cable.
Although the ribbon may contain many fibers, smaller groups of fibers may need to be dropped off at a node. This is particularly true in optical fiber network architectures, such as a fiber-in-the-loop (FITL) architecture. Some specific applications require accessing each individual optical fiber within a given ribbon. Such applications typically are referred to as Fiber To The x (FTTx) applications, with more common ones being Fiber To The Premises (FTTP) or Fiber To The Home (FTTH). In FTTx installations, individual optical fibers need to be accessed and provided to an end-user of a given premises (e.g. an apartment or an office at a given floor of a given building).
However, current ribbon structures with a single application of matrix material do not allow for easily splitting the ribbon into smaller subgroups or units. Instead, an attempt to split the ribbon can lead to a common phenomenon called “fiber fallout,” wherein the matrix material breaks and has insufficient adhesion with the fibers adjacent to where the split occurs to hold them together. These fibers adjacent to the split are often referred to as “border fibers.” As a result of fiber fallout, splicing problems arise since the fiber that has “fallen out” can't be used as a ribbon or spliced as part of a ribbon.
To counteract fiber fallout, splittable ribbons can be made by preforming the ribbon into subunits. This process uses two or more matrix layers. One matrix material binds several groups of fibers together as one or more subunits, while a second layer of matrix material encapsulates the entire ribbon, including the subunits. The two fiber subunits may be later separated from each other by breaking the outer matrix material (i.e., the second layer of matrix material). For example, a 24-fiber ribbon can be formed by joining two 12-fiber ribbons in a first matrix layer and then binding those two subunits with an outer matrix casing. Analogously, a 12-fiber ribbon can be formed by applying a second layer of matrix material over three 4-fiber subunits bound by first matrix layers.
Such a technical solution is disclosed, for instance, in U.S. Pat. No. 6,175,677 where a primary matrix ribbonizing layer envelopes the optical fibers of a subunit and a secondary matrix ribbonizing layer envelopes the primary matrix ribbonizing layers of all the subunits forming the optical fiber ribbon.
These dual-matrix-layer solutions are attractive when the size of the subunits to be separated from the fiber ribbon is predetermined. However, a more complex manufacturing process is required to produce a dual-matrix-layer ribbon with subunits. In fact, in order to make a 12-fiber ribbon by joining three 4-fiber subunits, the manufacturing process requires four passes on the matrix coating line (one for joining each of the 4-ribbon subunits in an inner matrix material and one for joining the subunits in an outer matrix material). On the other hand, a conventional 12-fiber ribbon with a single matrix layer requires only a single pass on the coating line.
Dual-matrix-layer ribbons with subunits have other disadvantages. For example, when a fiber ribbon is formed from subunits or subgroups, each subunit's matrix layer not only sandwiches the fibers of the subunit, it also fills the space on the lateral ends of the subunit, separating axially adjacent subunits from each other. This separation between subunits introduces inconsistent distances between optical fibers in the ribbon. In particular, adjacent fibers within each subunit contact each other, while adjacent fibers between subunits are separated from each other by matrix material. As most ribbon splicers require optical fibers to be side-by-side (i.e., substantially touching each other), the inconsistent distances in dual-matrix-layer ribbons negatively affect the splicing operation. In particular, splice losses increase due to the non-uniform location of the optical fiber cores, which are not correctly lined up within the ribbon.
Another technique for making the ribbon splittable involves adding at least one stress concentration, or area of weakness, to the matrix layer immediately surrounding the fibers. These stress concentrations help isolate a split in the ribbon to a particular location and facilitate separation of the optical fiber ribbon subunits. U.S. Pat. No. 5,717,805, for example, discloses an optical fiber ribbon provided with at least one stress concentration extending along at least a portion of the ribbon parallel to the ribbon longitudinal axis. The stress concentration(s) can be formed on at least one extreme edge of the optical fiber ribbon such that the matrix material may be easily removed from a section of the optical fiber ribbon at a desired ribbon access location. Alternatively, at least one stress concentration can be formed on at least one of the major surfaces of the optical fiber ribbon at designated locations where it is desired for the optical ribbon to be separated into smaller ribbon units. When stress concentrations are formed on the major surfaces of the optical fiber ribbon, additional matrix material is provided between adjacent optical fibers at the stress concentration location such that when an optical fiber ribbon is separated at the stress concentration location, complete sub-ribbons (sub-groups) are formed (see in particular FIGS. 5, 8 and 9 of U.S. Pat. No. 5,717,805). Other art, such as U.S. Pat. No. 5,982,968; U.S. Pat. No. 6,748,148; U.S. Pat. No. 6,792,184; U.S. Pat. No. 6,337,941; U.S. Pat. No. 7,085,459; and U.S. Pat. No. 7,039,282, also discloses the formation of at least one stress concentration in the optical fiber ribbon.
Similarly, U.S. Pat. No. 7,187,830 is concerned with optical fiber ribbon units with a preferential tear portion formed between adjacent fibers in an optical fiber ribbon unit. The tear portion is formed by a weakened portion of the ribbon matrix material, the weakened portion of the matrix having a reduced cure level compared with the surrounding matrix material, thereby creating the weakened portion. The reduced cure level in the matrix material is accomplished by varying the intensity of the radiation dose to cure the matrix material at the preferential tear location.
Applicants have observed that optical fiber ribbons having a single matrix layer are particularly preferred since the above mentioned drawbacks of the dual-matrix-layer solutions are advantageously avoided and, moreover, a ribbon with a single matrix layer has a favorable reduced size with respect to a ribbon having two distinct matrix layers.
However, Applicants have also observed that with optical fiber ribbons having a single matrix layer, the border fibers in the ribbon can lose adhesion from the matrix when the ribbon is split into smaller ribbon units and the fiber fallout phenomenon can occur. Moreover, this drawback can be accentuated when stress concentrations are incorporated in the ribbon to be split. For instance, if the stress concentration is a groove and the groove is too deep, the ribbon may be mechanically unstable when handled. On the contrary, if the groove is not deep enough, the ribbon matrix may break anywhere, including at a point where there is not enough matrix material to hold the end fiber with the other fibers of the ribbon subunit which has been formed (thereby resulting in fiber fallout).
In view of these shortcomings, a need exists for providing a splittable optical fiber ribbon split that reduces the occurrence of fiber fallout. A need also exists for providing a splittable optical fiber ribbon that can be easily and correctly split into two or more ribbon subunits. Furthermore, a need also exists for more efficient manufacturing processes for making splittable optical fiber ribbons with reduced occurrence of fiber fallout.